1. After the demise of your mother, your father inherited 1/6th of your demised mother's property,
2. He is very much within his right to will it in favour of any body he wishes to which can not be agitated upon legally,
3. However, if the signature of the Will is found to be forged or obtained by coercion or fraud, then the said will can be challenged by other legal heirs,
4. Your brother shall have to apply for probate of the Will of your father for which you will be posted with the required notice by the Court,
5. So, while replying the said notice you can file appropriate objection to the application for probate filed by your brother.